1. Field of the Invention
The present invention relates to a method and device for adjusting a multi-beam source unit used in an image forming apparatus such as a digital copying machine or a laser printer, as well as a method for assembling the multi-beam source unit and an image forming apparatus using the multi-beam source unit.
2. Description of the Prior Art
Heretofore, as an image forming apparatus such as a digital copying machine or a laser printer there has been known an image forming apparatus with a laser scanning optical system mounted thereon. With the recent tendency to a higher write accuracy and a higher write speed, a laser scanning optical system using a multi-beam laser diode is becoming most popular.
FIG. 1 illustrates such a laser scanning optical system schematically. In the same figure, the reference numeral 1 denotes a multi-beam source unit, numeral 2 denotes a polygon mirror, numeral 3 denotes a fθ lens, and numeral 4 denotes a photosensitive member (also called an image recording medium). The multi-beam source unit 1 substantially comprises a multi-beam laser diode 5 and a collimator lens 6. The multi-beam laser diode 5 has a plurality of light emitting points to emit multi-laser beams P. The multi-laser beams P are collimated by means of the collimator lens 6, then are reflected by the polygon mirror 2 and are conducted onto a surface (also called an image recording surface) 4a of the photosensitive member 4.
The polygon mirror 2 and the fθ lens 3 constitute a part of a scanning optical system. On the surface 4a of the photosensitive member 4, as shown in FIG. 2, the multi-laser beams P are scanned in a horizontal direction Q1 while leaving predetermined pitches X1 in a vertical scanning direction Q2 which is perpendicular to the horizontal scanning direction Q1. In this type of a laser scanning optical system, the surface 4a of the photosensitive member 4 is scanned over a large number of lines at a time to write data onto the surface 4a. 
As the write accuracy and speed have recently become higher, the laser scanning optical system is required to be improved in its accuracy for the diameter of a beam spot 11 of each multi-laser beam P, collimatability of the beams, a pitch X1 between adjacent beam spots 11 in the vertical scanning direction Q2, and a write start position in the horizontal scanning direction Q1. The accuracy required is becoming more and more strict as a higher image quality is required.
As shown in FIG. 3, the multi-beam laser diode 5 has a light emitting section 7 in the interior thereof. In the light emitting section 7 are provided a plurality of light emitting points, which are, for example, four light emitting points 7a to 7d. The light emitting points 7a to 7d are arranged on a designwise predetermined virtual straight line Q3 spacedly from one another. The virtual straight line Q3 is obtained by joining acute points 9a and 10a of a pair of acute-angled cutout portions 9 and 10 which are formed in a metallic stem 8 of the multi-beam laser diode 5.
In the conventional multi-beam laser diode 5, the light emitting points 7a to 7d are spaced widely from one another, so that when multi-laser beams are projected onto the surface 4a of the photosensitive member 4, the pitches X1 of their beam spots 11 in the vertical scanning direction Q2 become large and the image quality becomes coarse. For avoiding such an inconvenience, as shown in FIG. 4, the multi-beam laser diode 5 is turned around the optical axis of the scanning optical system (not shown) so that an arranged direction (straight line) Q3′ of the beam spots 11 becomes oblique with respect to the horizontal scanning direction Q1 on the surface 4a of the photosensitive member 4, thereby adjusting the pitches X1 in the vertical scanning direction Q2 to enhance the write density (recording density) in the vertical scanning direction Q2 and improve the image quality.
However, if the multi-beam laser diode 5 is turned so that the arranged direction (straight line) Q3′ of the beam spots 11 is deviated obliquely with respect to the vertical scanning direction Q2, to improve the write density, then in the case where the light emitting points 7a–7d are driven simultaneously to effect write, this results in that write start positions in the horizontal scanning direction Q1 of the beam spots 11 on the surface 4a of the photosensitive member 4 become displaced and hence the image quality is deteriorated.
In the laser scanning optical system of this type, for making the write start positions of the beam spots on the surface 4a of the photosensitive member 4 uniform, for example sensors 12 for detecting scan positions of the laser beams are arranged correspondingly to the laser beams and the emission of beam from each of the light emitting points 7a to 7d is controlled in accordance with a light receiving timing of each sensor 12.
More particularly, the emission of light from the light emitting points 7a–7d is controlled upon the lapse of time t0′ after the detection of the head beam spot 11 at time t=t0, thereby making write start positions in the horizontal scanning direction Q1 on the surface 4a of the photosensitive member 4 uniform.
Moreover, instead of using sensors 12 correspondingly to the light emitting points 7a–7d a sensor 12 is provided correspondingly to the head beam spot 11 in the horizontal scanning direction Q1, as shown in FIG. 5(a), then time lags t1, t2, and t3 of beam spots 11 are determined in advance, and as shown in FIG. 5(b), after the head beam spot 11 has been detected by the sensor 12, the emission of beams from the remaining light emitting points 7b–7d is delayed correspondingly to the time lags by means of a delay control circuit (not shown), thereby making the beam spots 11 uniform in the vertical scanning direction at the write start positions on the surface 4a of the photosensitive member 4, as shown in FIG. 5(a).
In this laser scanning optical system, however, the control circuit used for aligning write start positions is complicated, with consequent increase of cost.
At present, a multi-beam laser diode 5 with light emitting points 7a–7d spaced more narrowly than before is being developed. In a multi-beam source unit having such a multi-beam laser diode 5 it is presumed that positional variations of the light emitting points 7a–7d will be small. Further, the multi-beam source unit is probably designed so as to be set to a scanning optical system on the assumption that light emitting points 7a–7d are arranged in the direction of a predetermined standard design line when they are present on the virtual straight line Q3 defined by a pair of cutout portions 9 and 10, and is attached as it is to a body portion of an image forming apparatus.
Even with such a design, however, due to errors in the manufacturing process of the multi-beam laser diode 5 being considered, it is rarely the case that the light emitting points 7a–7d are positioned on the virtual straight line Q3 without error. Even if there exists an arrangement direction (straight line) Q4 joining light emitting points 7a–7d, as shown in FIG. 6, the arrangement direction Q4 and the virtual straight line Q3 are slightly inclined with respect to each other and thus aligning the arrangement direction of the light emitting points 7a–7d with the direction of the standard design line without the need of any adjustment is difficult. The reference mark θ represents the angle of that inclination.
When the multi-beam laser diode 5 is to be mounted to a body portion of an image forming apparatus body with a scanning optical system mounted thereon, since there exists a mounting error, it is desirable to turn the multi-beam laser diode 5 around the optical axis so that the arrangement direction Q4 of the light emitting points is at a predetermined angle relative to the vertical scanning direction Q2.